Composite anisotropy

Technically relevant applications are hardly concerned with complete anisotropy. Composites with a regular distribution of constituents along the principal axes are an example of a material with three orthogonal planes of symmetry. The description of such orthotropic properties requires nine independent matrix entries ... 

The mechanical piopeities of stmctuial foams and thek variation with polymer composition and density has been reviewed (103). The variation of stmctural foam mechanical properties with density as a function of polymer properties is extracted from stress—strain curves and, owkig to possible anisotropy of the foam, must be considered apparent data. These relations can provide valuable guidance toward arriving at an optimum stmctural foam, however. 

Anisotropy in metals and composite materials is common as a result of manufacturing history. Anisotropic materials often display significantly different results when tested along different planes. This appHes to indentation hardness tests as well as any other test. 

Change of compensation temperature with composition is Positive value of K indicates perpendicular anisotropy. 

None of the biaary compounds with this composition is well matched to the needs of MO recording. Gd—Fe has too high a Curie temperature and has an in-plane anisotropy. Tp is too low for binary alloys such as Tb—Fe and Dy—Fe. Co-based alloys which exhibit a close to room temperature have... 

Coercivity of Thin-Film Media. The coercivity ia a magnetic material is an important parameter for appHcations but it is difficult to understand its physical background. It can be varied from nearly zero to more than 2000 kA/m ia a variety of materials. For thin-film recording media, values of more than 250 kA / m have been reported. First of all the coercivity is an extrinsic parameter and is strongly iafluenced by the microstmctural properties of the layer such as crystal size and shape, composition, and texture. These properties are directly related to the preparation conditions. Material choice and chemical inborn ogeneties are responsible for the Af of a material and this is also an influencing parameter of the final In crystalline material, the crystalline anisotropy field plays an important role. It is difficult to discriminate between all these parameters and to understand the coercivity origin ia the different thin-film materials ia detail. 

In the Ee—Al—Si ternary system, alloys close to the 9.5 Si, 5.6 A1 composition exhibit very low magnetostriction and anisotropy. As a result, these show very high values of initial and maximum permeabiHty. However, the ternary alloys are very britde, a factor which limits their general usefiilness. 

Optimum permeability is achieved by choosing the composition where the anisotropy constants X are near zero, by using high purity raw... 

The two estimates, if plotted, look as shown in Fig. 6.4. This explains why fibre-reinforced composites like wood and GFRP are so stiff along the reinforced direction (the upper line of the figure) and yet so floppy at right angles to the direction of reinforcement (the lower line), that is, it explains their anisotropy. Anisotropy is sometimes what you want - as in the shaft of a squash racquet or a vaulting pole. Sometimes it is not, and then the layers of fibres can be laminated in a criss-cross way, as they are in the body shell of a Formula 1 racing car. 

Spruce soundboards have a Young s modulus anisotropy of about (11.6 GPa/0.71 GPa) = 16. A replacement material must therefore have a similar anisotropy. This requirement immediately narrows the choice down to composites (isotropic materials like metals or polymers will probably sound awful). 

Fig. 28.21. Modulus anisotropies for aligned GFRP and CFRP composites.

The inherent anisotropy (most often only orthotropy) of composite materials leads to mechanical behavior characteristics that are quite different from those of conventional isotropic materials. The behavior of isotropic, orthotropic, and anisotropic materials under loadings of normal stress and shear stress is shown in Figure 1-4 and discussed in the following paragraphs. 

Composite materials have many distinctive characteristics reiative to isotropic materials that render application of linear elastic fracture mechanics difficult. The anisotropy and heterogeneity, both from the standpoint of the fibers versus the matrix, and from the standpoint of multiple laminae of different orientations, are the principal problems. The extension to homogeneous anisotropic materials should be straightfor-wrard because none of the basic principles used in fracture mechanics is then changed. Thus, the approximation of composite materials by homogeneous anisotropic materials is often made. Then, stress-intensity factors for anisotropic materials are calculated by use of complex variable mapping techniques. 

Such oriented composites should also have electrical anisotropy. Indeed, for the composite material with

sample amounts to 5 x 10 5 Ohm-1 cm-1 which is also below... 

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